The present relates to the general field of devices for adjusting a fluid flow, and it relates more particularly to fuel injectors in a turbomachine combustion chamber.
The rate at which fuel is fed into a turbojet or a turboprop (referred to below as a xe2x80x9cturbomachinexe2x80x9d) is adjusted by means of injectors which are designed to deliver a first flow of xe2x80x9cmainxe2x80x9d fuel while the turbomachine is in a starting stage and when operating at low power (xe2x80x9cidlingxe2x80x9d), and a second flow of xe2x80x9csecondaryxe2x80x9d fuel during subsequent stages of operation all the way to full power.
French patent application No. FR 2 540 186 illustrates a typical example of a fuel injector for a turbomachine that comprises firstly a stop valve whose spring is adjusted in such a manner as to open at a predetermined fuel feed pressure corresponding to the starting stage and to remain open above said predetermined pressure, and secondly a metering valve whose spring is adjusted to open at a feed pressure that is also predetermined but higher than the starting pressure, and that remains open at higher pressures, i.e. up to the maximum pressure of use. During the second stage, fuel flow rate is adjusted by grooves machined in the metering valve and having flow sections that are designed to track the progressive variation in flow rate in application of a predetermined relationship depending on the feed pressure. The initial loading of the spring defines a first action threshold for the metering valve and is set by means of an annular spacer.
That prior art nevertheless suffers from a major drawback. The flow rate of fuel that corresponds to the metering valve being fully open (when the engine is at full power) depends essentially on the mechanical characteristics of the spring and on the flow sections of the grooves (friction in the valve is also involved but to a much lesser extent). Unfortunately, the tolerances that exist in the manufacture or machining both of the spring and of the grooves are such that this maximum flow rate cannot be identical for all of the injectors of a given engine. FIG. 6 shows the spread of flow rate differences xcex94Q1, xcex94Q2 that can exist between two different injectors 100, 102 in the same combustion chamber as compared with a calculated ideal flow rate 104. This results in great non-uniformity of flow rate between injectors which is particularly harmful to proper operation of the turbomachine.
An object of the present invention is thus to provide a fuel injector which makes it possible to reduce flow rate non-uniformity between the various injectors in a common combustion chamber to a considerable extent. Another object of the invention is to provide a device which makes it possible for the adjustment of fuel injection flow rates into the combustion chamber to be made simpler and more reliable for each of the injectors. Another object of the invention is to provide a device which also makes it possible to save time when individually adjusting these flow rates. A further object of the injector is to provide a device that is capable of overcoming friction in the metering valve of such an injector.
These objects are achieved by a fuel injector for a turbomachine combustion chamber, the injector comprising an injector body having means for admitting fuel under pressure, a first valve mounted downstream from said means for admitting fuel under pressure and responding to a predetermined fuel admission pressure to admit fuel into said injector body, a second valve mounted downstream from said first valve and responding to a first determined fuel pressure S1, greater than said admission pressure to deliver a fraction of the fuel admitted into said injector body to fuel utilization means, the second valve metering the flow rate of fuel injected to said utilization means up to a second determined fuel pressure S2, greater than said first pressure, above which said second valve behaves like a fixed diaphragm as a function of the flow sections formed through said second valve, the injector further comprising means for individually adjusting said second determined fuel pressure so as to ensure that fuel is injected at a uniform rate into the combustion chamber.
With this particular structure, and at high flow rates (above the second determined pressure S2), any dispersion in the flow rate characteristics of the utilization means is avoided and adjustment of the maximum flow rate at which fuel can be injected is made considerably simpler.
Advantageously, the adjustment means comprise an abutment for limiting the stroke of said metering valve to a determined value which corresponds to a desired maximum value for the injection flow rate of fuel, and said stroke-limiting abutment preferably comprises a ring fixed on said metering valve for the purpose of co-operating with a bushing in which said valve can move.